Here is a document with some interesting FFTs. They are of a 300B output showing the spectral response.

Plot 1 is particularly interesting as it shows (1) second harmonic is less than 3dB greater than 3rd Harmonic, (2) there are measurable harmonics out to 24th shown, and they probably exist beyond that.

Plots 4 and 5 are more in line with traditional thinking in that 2nd harmonic is greater than third. However, the high order harmonics are still present.

The driver is a 6SN7, so there is no doubt that Triodes generate odd harmonics.

The 2HD relative to 3HD level is dependent on the operating point and how hard the amplifier is being driven. It looks like you are driving it quite hard or using a non-optimal operating point because there are lots of harmonics visible above 9th. A nicely cruising valve amplifier will show a steady decay of 2HD > 3HD > 4HD > 5HD. Harmonics beyond that are usually buried in the noise floor.

Here is a document with some interesting FFTs. They are of a 300B output showing the spectral response.

Plot 1 is particularly interesting as it shows (1) second harmonic is less than 3dB greater than 3rd Harmonic, (2) there are measurable harmonics out to 24th shown, and they probably exist beyond that.

Plots 4 and 5 are more in line with traditional thinking in that 2nd harmonic is greater than third. However, the high order harmonics are still present.

The driver is a 6SN7, so there is no doubt that Triodes generate odd harmonics.

There must be something else going on here. We are not seeing just the distortion of the output tube. The transfer function shape of ALL triodes is similar and for normal excursions within this function 2H dominates with other harmonics at least 10dB below. The only time this does not occur is when you explore the extremities of the characteristic when all the harmonics pop up in greater proportions. I find it very hard to believe that the harmonic sequence illustrated can occur where the second harmonic is itself 50dB below the fundamental. The only simple thing I can think of is the 2H from the 6SN7 driver is cancelling some of the 2H from the 300B.

The circuit on the associated web page is rather strange. The first half of the 6SN7 is run at a fairly standard 4mA plate current and is directly coupled to the second triode which drives the 300B. The driver stage appears to be run at about 4.5mA which I would suggest is perhaps a little low for this application.

Working backwards from the 8 ohm speaker the primary impedance is 3K so the transformer turns ratio is about 19:1. For 1W into 8 ohms there needs to be 2.828V rms on the secondary or about 55Vrms at the plate. If we are generous and allow the 300B stage a gain of 3 times this means there need to be about 18V rms at the 6SN7 plate. I know from numerous experiments that at this signal level, the intrinsic 2H distortion of a 6SN7 CC stage is somewhere between 0.2 and 0.4% or around -50dB or so. That's probably just about the right level to cancel much of the 300B stage 2H as indicated by the measurements.

I have another question about SE distortion. I know that IM distortion increases as power output increases. But does harmonic distortion increase in the same way? I'm designing an SE amp using 6688 preamp and a 6550 output tube. The amp should put out about 20 watts, hopefully. TDSL lists harmonic distortion for 6550 at 2% or less for PP designs but doesn't give a spec for SE. Anyone no what it is? And will it become unnacceptable at 20 watts?

I have another question about SE distortion. I know that IM distortion increases as power output increases. But does harmonic distortion increase in the same way? I'm designing an SE amp using 6688 preamp and a 6550 output tube. The amp should put out about 20 watts, hopefully. TDSL lists harmonic distortion for 6550 at 2% or less for PP designs but doesn't give a spec for SE. Anyone no what it is? And will it become unnacceptable at 20 watts?

I have another question about SE distortion. I know that IM distortion increases as power output increases. But does harmonic distortion increase in the same way? I'm designing an SE amp using 6688 preamp and a 6550 output tube. The amp should put out about 20 watts, hopefully. TDSL lists harmonic distortion for 6550 at 2% or less for PP designs but doesn't give a spec for SE. Anyone no what it is? And will it become unnacceptable at 20 watts?

Yes THD increases with output power, @20W, THD=13.5%, is that too much

IM and harmonic distortion come from the same source: device nonlinearity. Except where there is some cancellation you can expect them both to vary in similar ways with signal level. When there is cancellation then there will be higher order terms arising from that cancellation, in exactly the same way as those which arise from feedback - except that cancellation distortion only goes up to 2N in order, when each stage goes up to N in order.